1. Field of the Invention
This invention relates to a method and apparatus to measure a gas mass fraction.
2. Discussion of the Background
The numerical value of the gas mass fraction measured may be used in a calculation to give a numerical value of a gas mass flow rate in a multiphase flow, for example a wet gas flow comprising gas and liquid. Typically, wet gas comprises 90% or more of gas by volumetric flow under flow conditions.
The wet gas concerned may be fuel gas, for example natural gas, and liquid, for example oil and/or water, being piped from an oil or gas well.
Hitherto, the measurement of fluid flow from an oil or gas well has usually involved the use of a test separator to devise a value for the gas mass fraction which is the ratio of the gas mass flow to a total fluid (gas and liquid) flow. Test separation which is a batch process, is performed offshore in offshore oil and gas production, and if need for test separation can be obviated this can reduce offshore platform weight, process complexity and the manning requirement demanded by test separation.
An alternative to test separation is the use of expensive multi-phase flow meters, but these lose accuracy as the gas volume fraction increases.
An alternative to such multi-phase flow meters is the use of a venture for wet gas venturi flow metering in which a correction using either the Murdock or Chisholm formula is applied thereto to give the mass flow rate of the gas. However both the Murdock and Chisholm formulas require a derived value of the gas mass fraction proved by test separation using the test separator as referred to above, or by using a tracer technique which is also a batch process, or by gamma densitometry which is a continuous process.
An object of the invention is to provide a method of measuring a gas mass fraction in a mass of liquid and gas flowing along a pipeline, which may be a continuous process which when carried out avoids the need for test separation or for use of a tracer technique, or for use of gamma densitometry.
According to a first aspect of the invention a method of measuring a gas mass fraction X in a mass of liquid and gas flowing along a pipeline comprises providing a flow conditioner in the pipeline, providing adjacent to and downstream of the flow conditioner a venturi flow meter in-line with the pipeline so that fluid flow along the pipeline is through the flow conditioner and the venturi flow meter, making measurements of differential pressures across a plurality of portions along the flow conditioner and venturi flow meter combination, and using the measurements to calculate the gas mass traction X.
According to a second aspect of the invention a method of measuring a gas mass fraction X in a mass liquid and gas flowing along a pipeline comprises
providing a venturi comprising a throat, an upstream portion converging towards the throat, and a downstream portion diverging away from said throat.
providing said venturi in-line with the pipeline such that fluid flow along the pipeline is through the venturi,
providing a flow conditioner in the pipeline upstream of the venturi, said flow conditioner being adjacent to said venturi,
making a measure of a differential pressure DP1 between a position upstream of the flow conditioner and a position intermediate the flow conditioner and the venturi,
making a measure of a differential pressure DP2 between the throat and position intermediate the flow conditioner and the venturi,
making a measure of a differential pressure DP3 between a position downstream of the venturi and a position intermediate the flow conditioner and the venturi, and
calculating the gas mass fraction X using a formula of the type: (X=fn(DP1,DP2,DP3 . . . ) of which X=axc2x7(DP1)bxc2x7(DP2)cxc2x7(DP3)dxc2x7(DP2xe2x88x92DP3)exc2x7(DP1+DP3)f, in which a,b,c,d,e and f are constants, is one example